Field of the Invention
The present invention relates to an imaging optical system such as a zoom lens, and more particularly to an imaging optical system which is suitably used for an image projection apparatus that enlarges and projects an image.
Description of the Related Art
Recently, a high reality and high presence are required resulting from a super resolution of an image, and in order to reproduce them, an imaging optical system having a high resolution performance is desired. Furthermore, an imaging optical system which has an image plane with a higher flatness and a smaller chromatic aberration of magnification is required due to a miniaturization of pixels and a decrease of a permissible width of depth by enhancing the resolution of the imaging optical system. In addition, characteristics in which the flatness of the image plane and the chromatic aberration of magnification do not vary during focusing from an infinity to a close range are required.
In an image projection apparatus such as a projector, it is necessary to provide a space to guide illumination light to an image formation element, and accordingly a back focus of a lens unit needs to be long to some extent. In the image projection apparatus, a pupil is determined by an illumination optical system. In order to ensure a satisfactory illuminance distribution, the lens unit needs to be telecentric at its image side. It is preferred that distortion of the lens unit is appropriately corrected so that distortion does not occur in a projected image with respect to an original image.
Japanese Patent Laid-open No. 2003-222793 discloses an image pickup lens in which a first lens unit is divided into three lenses of a negative lens, a negative lens, and a positive lens and focusing from an infinity to a close range is performed by moving a middle lens unit having a negative refractive power to an enlargement conjugate side. Japanese Patent Laid-open No. 2011-186269 discloses a projection wide-angle lens in which a first lens unit is divided into a negative lens, a positive lens, and a positive lens, and focusing from the infinity to the close range is performed by moving a middle lens unit having a positive refractive power to the enlargement conjugate side and also by moving a lens unit closest to a reduction conjugate side to the reduction conjugate side.
However, in the image pickup lens disclosed in Japanese Patent Laid-open No. 2003-222793 and the projection wide-angle lens disclosed in Japanese Patent Laid-open No. 2011-186269, a power balance of dividing the first lens unit is not optimum, and accordingly it does not have a sufficient performance if extremely-high resolution is required. Furthermore, both of Japanese Patent Laid-open No. 2003-222793 and Japanese Patent Laid-open No. 2011-186269 relate to a fixed focal lens, and therefore factors affected by zooming are not considered.